Plate Tectonics and Volcanoes

As with earthquakes, volcanic activity is linked to plate-tectonic processes. Most of the world''s active above-sea volcanoes are located near convergent plate boundaries where subduction is occurring, particularly around the Pacific basin.

However, much more volcanism -- producing about three quarters of all lava erupted on Earth -- takes place unseen beneath the ocean, mostly along the oceanic spreading centers, such as the East Pacific Rise.

Subduction-zone volcanoes like Mount St. Helens (in Washington State) and Mount Pinatubo (Luzon, Philippines), are called composite cones and typically erupt with explosive force, because the magma is too stiff to allow easy escape of volcanic gases.

Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from those of composite cones.

Mauna Loa and Kilauea, on the island of Hawaii, are known as shield volcanoes, because they resemble the wide, rounded shape of an ancient warrior''s shield. Shield volcanoes tend to erupt non-explosively, mainly pouring out huge volumes of fluid lava. Hawaiian-type eruptions are rarely life threatening because the lava advances slowly enough to allow safe evacuation of people, but large lava flows can cause considerable economic loss by destroying property and agricultural lands. For example, lava from the ongoing eruption of Kilauea, which began in January 1983, has destroyed more than 200 structures, buried kilometers of highways, and disrupted the daily lives of local residents. Because Hawaiian volcanoes erupt frequently and pose little danger to humans, they provide an ideal natural laboratory to safely study volcanic phenomena at close range. The USGS Hawaiian Volcano Observatory, on the rim of Kilauea, was among the world''s first modern volcano observatories, established early in this century.

Continental" Volcanoes
In the typical "continental" environment, volcanoes are located in unstable, mountainous belts that have thick roots of granite or granitelike rock. Magmas, generated near the base of the mountain root, rise slowly or intermittently along fractures in the crust.

During passage through the granite layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of nonbasaltic rocks

"Island-Arc" Volcanoes
In a typical "island-arc" environment, volcanoes lie along the crest of an arcuate, crustal ridge bounded on its convex side by a deep oceanic trench. The granite or granitelike layer of the continental crust extends beneath the ridge to the vicinity of the trench.

Basaltic magmas, generated in the mantle beneath the ridge, rise along fractures through the granitic layer. These magmas commonly will be modified or changed in composition during passage through the granitic layer and erupt on the surface to form volcanoes built largely of nonbasaltic rocks.

"Oceanic" Volcanoes
In a typical "oceanic" environment, volcanoes are alined along the crest of a broad ridge that marks an active fracture system in the oceanic crust. Basaltic magmas, generated in the upper mantle beneath the ridge, rise along fractures through the basaltic layer.

Because the granitic crustal layer is absent, the magmas are not appreciably modified or changed in composition and they erupt on the surface to form basaltic volcanoes

C http://vulcan.wr.usgs.gov/Glossary/VolcanoTypes/island_oceanic_continental.html

Volcano Types Island-Arc, Oceanic, and Continental Volcanoes
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